Plans for the invasion of England continued to be made throughout the summer of 1940. On 1st August, Hitler ruled that all Services should be ready to launch the operation on 15th September; on 3rd September it was postponed till 21st September. But the essential precondition of air superiority was never obtained; the great air battles over Britain, culminating in the battle of 15th September, convinced Hitler that invasion was impossible; and on 17th September he gave orders that the operation be indefinitely but inconspicuously postponed. By the time that the second winter of war approached, Hitler had thus failed to secure his huge conquests; nor did he feel that they would be secure until Britain had been crushed. This failure was particularly galling to him because he already felt the approach of Russian power in the east, from Finland to Romania. The longer he found himself committed to war with Britain, the more he feared that his initial conquests in the east, which were to be the basis of a longer policy of conquest, would be undermined in his rear. It was therefore essential to him to find some means of bringing Britain to reason; and if that was impossible by direct attack, he must seek to achieve it by indirect means.
Fortunately such means, he thought, lay ready to hand. Italy had declared war on Britain and France in the hour of Hitler's victory, on 10th June. On 13th September Mussolini ordered the invasion of Egypt from Libya; on 15th October he also invaded Greece. Both invasions were unsuccessful. Meanwhile, General Franco, in Spain, was showing himself anxious to profit from Hitler's patronage, though reluctant to pay any price for it. Mussolini's necessities and Franco's appetite suggested new projects to Hitler. A winter campaign in the warm Mediterranean Sea area might cut Britain off from other continents, just as the summer campaign in the north had cut it off from Europe. Thus Britain would be completely isolated and, in isolation, might be strangled, since it would not be beaten, into submission. Directive No. 18 reviews these possibilities.